As known, a bicycle wheel rim comprises a radially outer portion configured to be coupled with a tire and a radially inner portion comprising a plurality of attachment holes for respective spokes.
Typically, the radially inner portion comprises opposite side walls and a bottom wall in which the aforementioned holes are formed.
“Symmetrical rims” and “asymmetrical rims” are known, that is rims whose radially inner portion has a radial section being respectively symmetrical or asymmetrical with respect to a sagittal middle plane of the rim.
In the present description and in the subsequent claims, the expression “sagittal middle plane” is used to indicate a plane perpendicular to the rotation axis of the rim and passing through the middle of the tire mounted on the rim. The sagittal middle plane thus coincides with the plane of symmetry of the tire mounted on the rim.
Typically, the spokes are associated with the wheel at the aforementioned holes and are oriented with a predetermined inclination with respect to the sagittal middle plane, so as to obtain a predetermined camber angle.
Symmetrical rims are frequently used for bicycle front wheels, whereas asymmetrical rims are often used for rear wheels. Indeed, the asymmetrical shape of the radially inner portion of the rim makes obtaining high camber angles easier, wherein high camber angles are required in particular for rear wheels due to the provision of the sprocket assembly on one side of the hub. Asymmetrical rims can, however, also be used in front wheels, particularly where disc brakes are used.
In the field of bicycles, in particular for racing bicycles, there is a particular need to manufacture rims that are strong and reliable, while at the same time being light.
Therefore, there is a tendency to make the walls of the radially inner portion of the rims with low thicknesses, such thicknesses still being sufficient to give the rim the desired characteristics of strength and reliability.
The Applicant has found that the head of the tightened spokes exerts a considerable stress on the rims in the contact area around the respective holes. Such stress determines a high concentration of tension at the holes, which involves the need to provide a predetermined thickness at least on the bottom wall.
EP 2422959 to the same Applicant discloses symmetrical rims provided with perforated inserts for holding the head of the spokes. The spokes pass through the inserts at respective holes made on a bottom wall of the rim. The inserts are arranged between the head of the spokes and the bottom wall, at the aforementioned holes.
The inserts described in EP 2422959 allow the stress generated by the tightened spokes to be distributed over a contact area around the holes of the bottom wall that is larger than that which there would be without inserts. Consequently, the thickness of the bottom wall can be advantageously lower than in the case in which the inserts are not used.
The Applicant has however found that the contact between the bottom wall and the insert can be improved. By improving the aforementioned contact, it is possible to improve the distribution over the rim of the stress generated by the tightened spokes, so as to further reduce the thickness of the bottom wall and, therefore, to further lighten the rim.
The Applicant has found that also in asymmetrical rims there is a need to reduce the thickness of the walls in order to lighten the rim and that also in this case such a requirement can be met by paying particular attention to the stresses generated on the rim by the attachment head for the tightened spokes.
The technical problem at the basis of the present invention is therefore to achieve an optimal distribution of tension over the rim (be it symmetrical or asymmetrical) of the tightened spokes, in order to be able to reduce the thickness of the rim and thus obtained a desired weight reduction.